The cornea is not only a major component of the optical system of the human eye, by providing about 75% of the total dioptric power, but also serves a protective function against a possible hostile environment. In order to perform normally its functions (refraction, transmission, protection), the cornea must actively maintain its transparency and integrity throughout life.
Replacement of an injured and/or opaque cornea with artificial materials has a long history of failure and even today its success is very limited.
As an alternative, the transplantation of human homologous corneal tissue is successful in less complex pathological conditions, such as keratocomus or corneal dystrophies. However, with gross scarring of the cornea, particularly when the host tissue is deeply vascularized, the tear film characteristics are altered, or secondary glaucoma is present, as may occur in conditions such as alkali burns, ocular pemphigoid, Stevens-Johnson syndrome, trachoma and other conditions, the clear graft rate drops significantly. Graft rejection or late graft failure decrease furthermore the chances of a successful transplantation. In addition, even in the countries with organized eye banking systems, there is a chronic shortage of donor corneal tissue. In the developing countries, less than 1% of the required transplants are carried out due to lack of availability of tissue and technology. If an artificial cornea (keratoprosthesis) was available with relatively simple means of fixation then millions more people worldwide might obtain visual rehabilitation.
The material almost exclusively used for keratoprostheses has been poly(methyl methacrylate), henceforth designated as PMMA. Despite a long recorded history, the success of various types of keratoprosthesis made from PMMA is still limited, mainly because of complications due to lack of healing at the interface between stromal tissue and peripheral prosthetic material, such as erosive tissue necrosis (melting), leakage of aqueous humor, epithelialization, infection, and extrusion of the implant.
Poly(2-hydroxyethyl methacrylate) hydrogel, henceforth designated as PHEMA, was another prosthetic material which received interest from ophthalmologists. Biocompatibility of PHEMA in the cornea is now well established, having been used as implant material for keratophakia (intracorneal lenses), as well as for epikeratoplasty. To a lesser extent, PHEMA has also been proposed as a material for keratoprostheses, showing good results in the animal models.
For many years, attempts have been made to use polymers such as PMMA, PHEMA, or other materials for keratoprosthetic implants. These attempts are well documented by the patent literature, for example in U.S. Pat. Nos. 2,517,523; 2,714,721; 2,754,520; 2,952,023; 3,454,966; 3,458,870; 3,945,054; 4,346,482; 4,402,579; 4,470,159; 4,586,929; 4,612,012; 4,624,669; 4,676,790; 4,693,715; 4,772,283; and 5,030,230, and in Ger. Pat. No. 2705234; Neth. Pat. No. 8501403; and Fr. Pat. No. 2,649,605. For general reviews covering the modern history of keratoprosthesis see: Day, R., Transactions of the American Ophthalmological Society, vol. 55, pp. 455-475 (1957), "Artificial corneal implants"; Stone Jr., W., Yasuda, H. and Refojo, M. F., "A 15-year study of the plastic artificial cornea-basic principles", in The Cornea World Congress, King Jr., J. H. and McTigue, J. W., eds., Butterworths, Washington, 1965, pp. 654-671; Polack, F. M., British Journal of Ophthalmology, vol. 55, pp. 838-843 (1971), "Corneal optical prostheses"; Mannis, M. J. and Krachmer, J. H., Survey of Ophthalmology, vol. 25, pp. 333-338 (1981), "Keratoplasty: A Historical Perspective"; Barron, B. A., "Prostokeratoplasty", in The Cornea, Kaufman, H. E., McDonald, M. B., Barron, B. A. and Waltman, S. R., eds., Churchill Livingstone, New York, 1988, pp. 787-803.
The postoperative complications of PMMA implants appear to be caused by the lack of a firm bond between the remaining corneal tissue and these implants. Prostheses have therefore been designed consisting of two distinct parts, namely a central optic cylindrical zone (PMMA), and a surrounding skirt made of materials different from PMMA such as metals, ceramics, preserved biological tissue and various polymers. However, all these modifications did not lower significantly the implant extrusion rate, as revealed in the above mentioned reviews. It therefore became clear that biocompatibility alone is not sufficient to overcome the problems of melting, leakage and extrusion. Ideally, the peripheral material should be incorporated into the host biological substrate by cellular invasion and growth across the interlace between material and tissue.